okey dokey! (dont worry, we all had to start somewhere)
you’ve got your footage. make a composition for it, but the footage in the composition.
select the layer. hit control+D and it will duplicate the layer. in your duplicate layer, find a frame that you want that the boom isn’t in. at that point, right click the layer and click Time>freeze frame.
then, go to the top and select the pen tool. this is your mask (rotoscoping) tool. clicking will add points. simply mask around the top of the image enough to adequately block the boom mic on the layer below it.
THEN, while the second layer is still selected. hit MM (m twice quickly) and you’ll get all the properties for the mask you just made, which includes feathering. feathering sort of blurs the edges. so up the value on this until no noticeable line is there anymore.
and voila! movie magic.
THE BiG HONKIN’
“The Defenders Of Stan”-lots of after effects